[docs] Add doc examples for DictReader and DictWriter (issue 20351)

[charlesaxel.dein at gmail.com]

http://bugs.python.org/review/20351/diff/11674/Doc/library/csv.rst
File Doc/library/csv.rst (right):

http://bugs.python.org/review/20351/diff/11674/Doc/library/csv.rst#newcode162
Doc/library/csv.rst:162: A short usage example::
You mean without `A short usage example`? I wanted to mimick the other
examples for `csv.reader`.

http://bugs.python.org/review/20351/